Method for actuating a holding brake

ABSTRACT

Based on the realization that, at the instant a holding brake in the drive of a machine tool is actuated, it may not be necessary to precisely observe the setpoint values, it may be provided to adapt control parameters in the controller of the drive such that, given an engaged holding brake, oscillations may be avoided or substantially reduced. By applying a correction value to the controller, it may be possible to prevent the adaptation of the control parameters from resulting in a change at the output of the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to application No. 102 29350.3, filed in the Federal Republic of Germany on Jun. 29, 2002, whichis expressly incorporated herein in its entirety by reference thereto.

FIELD OF THE INVENTION

[0002] The present invention relates to a method for actuating a holdingbrake. Such holding brakes are used in drives having electric motors, inorder, for example, to stop the axis driven by the motor when theelectric motor is de-energized and, therefore, is not able to assure areliable stoppage of the axis.

BACKGROUND INFORMATION

[0003] An example of an application area for holding brakes isnumerically controlled machine tools. For each of the axes of such amachine tool, a drive is provided, the motor of which is controlled viaa closed-loop control structure, including, for example, positionalcontrollers, speed controllers and current controllers. In this example,positional values are input by a numerical control and are thenconverted by the control structure into drive signals for the powercircuitry of the motor. In the control parameter setting, the decisionis made as to how precisely the drive, including the motor, powercircuitry, and control structure, realizes the inputs from the numericalcontrol. Thus, in a PI controller, it is, above all, the gain factors inthe proportional and integral parts of the controller that may beimportant. They determine the intensity of the system's reaction to adeviation from the particular setpoint value.

[0004] The drive of a machine tool is, in fact, able to hold an axis ina preset position. However, this is no longer effective when the driveis de-energized, for example due to a power failure. For that reason, onmachine tools, it is conventional to provide mechanical holding brakesas well, which are used in the currentless state. To release theseholding brakes, a current must flow, for example, through anelectromagnet. The need arises, above all, for suspension axes to bestopped via such holding brakes, since without drive control, a movementof the axis can be caused by the gravity of the suspended load.

[0005] If it is intended to stop an axis of a machine tool, then it maybe an important consideration that the time of the axis-drive controloverlap with the time in which the holding brake has already beenengaged. Otherwise, one risks a short time span in which the axis isneither stopped by the drive nor by a holding brake.

[0006] The control parameters of a drive are optimized for an operationwithout an engaged holding brake. In a machine tool, in particular, theparameters are often adjusted in a manner which will allow the pathdeviations of a tool to be corrected very aggressively, in order to keepfollowing errors or servo lag to a minimum. However, the conditionschange completely when the holding brake is engaged, since the motor ofthe drive now suddenly experiences a substantially higher load. Thisleads to undesirable oscillations in the drive, which can be perceivedby a whistling of the motor, for example. These oscillations are notonly unpleasant for the operator, they also subject the holding brake toincreased wear. It can even happen that the holding brake breaks loose,since the sliding or dynamic friction of a holding brake is distinctlyless than the static friction.

[0007] Japanese Patent Publication No. 2000-47732 describes, whenreleasing a holding brake, to switch over between different gain factorsin the control loop of a servo drive. By applying a high gain factorimmediately after the holding brake is released and by switching over toa lower gain factor some time later, on the one hand, an axis should beprevented from falling and, on the other hand, oscillations in normaloperation should be avoided. To avoid oscillations given asimultaneously active brake and active control, however, this procedureis not goal-directed when the brake is engaged, since it is precisely inthis case that high gain factors lead to vibrations.

[0008] If, as a general principle, the control parameters are selectedsuch that no oscillations occur even when the holding brake is engaged,then this has a negative effect on the quality of the control loop. Thesame applies to the use of filters in the control loop which avoid suchoscillations, even if they are also active when the holding brake is notactivated.

[0009] It is an object of the present invention to provide a method foractuating a holding brake which may enable oscillations to be avoided orgreatly reduced, given a simultaneously active holding brake and activecontrol.

SUMMARY

[0010] The above and other beneficial objects of the present inventionmay be achieved by providing a method as described herein.

[0011] At this point, to actuate a holding brake of an electric drivehaving a motor and control loop, it may be provided to adapt at leastone of the control parameters of the control loop in a first step suchthat, given an applied holding brake, oscillations may be avoided orgreatly reduced and, in a second step, to actuate the holding brake.

[0012] The foregoing is based on the realization that, on the one hand,given an activated holding brake, a high positioning accuracy may not berequired and, on the other hand, a reliable arresting of the driven axismay be possible, even given control parameters which are distinctlychanged in comparison to normal operation.

[0013] Tests have shown that excellent results may be attained byreducing the gain factors in the proportional and integral parts of aPI-controller, in particular of an rpm or speed controller, by a factorof between two and ten, approximately 100 ms prior to engaging theholding brake. In this manner, oscillations at the moment of the brakeapplication may be able to be nearly completely avoided.

[0014] When the switch is made to new control parameters, it may beassured that no change occurs at the output of the particular controlloop. A change may lead to unwanted and possibly dangerous movements ofthe drive. For that reason, a correction value may be fed to the controlloop to prevent any change at its output. This may be achieved in acontroller having an integral part by adding a correction value to theintegrator. This correction value is calculated such, for example, thatthe sum from, first of all, the deviation from the setpoint value,multiplied by the original proportional gain factor and, second of all,from the integrated deviation from the setpoint value, multiplied by theintegral gain factor, corresponds precisely to the sum from, first ofall, the deviation from the setpoint value, multiplied by the reducedproportional gain factor and, second of all, from the corrected contentof the integrator, multiplied by the integral gain factor. Since, givena reduction in the gain factors, the content of the integrator may beexpanded, this will be called a loading of the integrator.

[0015] The present invention may be able to be simply implemented usingdigital controllers, since, in this case, the integrator may be able tobe loaded by a simple addition in the memory register of the integrator.The correction value may be calculated, for example, in the software ofthe numerical control.

[0016] Besides the gain factors of the controllers in the control loop,other control parameters may also be adapted in the control loop. Asmentioned further above, filters may also be provided, the filterparameters of which, such as frequency, attenuation or time constant,may be effectively adapted to avoid oscillations. Here as well, animportant consideration may be that no change occur at the output of thecontrol loop which may be a sudden disturbance to the motor, and maythen have to first be compensated by the control again.

[0017] The method described for actuating a holding brake may also beused for releasing a holding brake. For this purpose, given an engagedholding brake, the controller of the drive is initially put in operationwith control parameters which may avoid or greatly reduce theoscillations. Only then is the holding brake released, and a short timelater, possibly in consideration of a suitable correction value, is theswitch made to the control parameters required for a high controlperformance.

[0018] The method according to the present invention may be suited forrotary drives and/or for linear drives.

[0019] Further aspects of the present invention and details pertainingthereto are derived from the following description of an exampleembodiment, on the basis of the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic view of a numerically controlled drivehaving a holding brake.

[0021]FIG. 2 is a schematic view of a control loop having a speedcontroller and current controller.

DETAILED DESCRIPTION

[0022] In FIG. 1, a numerical control NC is illustrated, which controlsa drive made up of a controller R, a power circuitry section L, and amotor M. Numerical control NC supplies a setpoint value Nsetpoint(Nsoll) for the speed of motor M to controller R. Controller R makesavailable voltage setpoint values Usetpoint (Usoll) for power circuitrysection L, which control motor M, for instance, via a pulse-widthmodulation. A tachometer monitors the speed of motor M and feeds back anactual value Nactual (Nist) of the speed to controller R. Currentsensors, which supply current actual values to the controller, are alsoprovided. A holding brake B is used for arresting the axis and may beactuated via numerical control NC.

[0023] In controller R, which is illustrated in greater detail in FIG.2, there is a PI controller for the speed and a current controller SR.This system may be used, for example, to drive a tool spindle.

[0024] If a positioning axis is to be driven, then controller R may besupplemented with a series-connected positional controller, whichreceives a positional setpoint value from numerical control NC and apositional actual value from a position sensor and supplies a speedsetpoint value Nsetpoint (Nsoll) to the speed controller.

[0025]FIG. 2 illustrates how a current setpoint value is generated forcurrent controller SR from the difference ΔN between speed setpointvalue Nsetpoint (Nsoll) and speed actual value Nactual (Nist). In theproportional part, difference ΔN is multiplied by a proportional gainfactor KP. In the integral part, difference ΔN is integrated as afunction of time in integrator I, and integral sum i is multiplied by anintegral gain factor KI. The sum of the two multiplications is appliedas current setpoint value Isetpoint (Isoll) at the output of the speedcontroller. As mentioned, from this, current controller SR generates avoltage setpoint value Usetpoint (Usoll), which is transmitted to powercircuitry section L.

[0026] To engage holding brake B, at this point, in a first step, atleast one control parameter is adapted in the control loop, here by aclear reduction in gain factors KP and KI. At this moment, it is nolonger a question of precisely adhering to the predefined speed, i.e.,to a good positioning accuracy. Thus, this reduction may not have anyadverse effect. Since by reducing the gain factors KP and KI, the outputof speed controller Isetpoint (Isoll) may change, a correction valueKorr may be fed, however, to the speed controller to prevent this.

[0027] This may be achieved most simply when integrator I is loadedaccordingly. Correction value Korr is able to be calculated, asdescribed above, from the condition that changing gain factors KP and KImay not result in a change at the output of the speed controller. If theoriginal gain factors are designated by KP1 and KI1, the reduced gainfactors by KP2 and KI2, then it may hold that:

ΔN*KP1+i*KI1=ΔN*KP2+(i+Korr)*K12

[0028] From this condition, a suitable correction value Korr is able tobe calculated in numerical control NC. This correction value keeps thecurrent-setpoint value Isetpoint (Isoll) constant in response to achange in gain factors KP and KI in the speed controller.

[0029] From the above relation one may also recognize that, in practice,prior to reduction of control parameters KP, KI, it may be beneficial towait for a compensated state, so that ΔN vanishes. Less of a correctionmay be needed in the described case. In other control systems, acorrection may even be entirely superfluous, for instance when in theintegral part of a PI controller, ΔN is already multiplied by a gainfactor KI, and this product is first integrated (thus, in FIG. 2,integrator I and amplifier KI are interchanged). At the output of thecontroller, a change in gain factors KP, KI, may not result, namely, ina change at ΔN=0. Gain factors KP, KI may be first adapted upon stoppageof motor M.

[0030] Holding brake B is first engaged in the second step, in practice,approximately 50-200 ms following reduction of gain factors KP and KI.At this point, oscillations of motor M caused by poorly adapted controlparameters may be reliably prevented, due to the reduced controlparameters.

[0031] In the method described herein, it may not necessarily be aquestion of the sequence of the two steps. A functional sequence is alsopossible, where holding brake B is first engaged, and, only then,possibly only in response to the occurrence of oscillations, are gainfactors KP, KI reduced. In some instances, however, additional outlaymay be required to detect the oscillations, and oscillations may nolonger be able to be completely avoided. When releasing holding brake B,it may be, however, practical to reverse the described sequence, sincecontroller R is put into operation here with reduced gain factors KP,KI, and higher gain factors KP, KI are not employed until after holdingbrake B is released. Here, as well, a change at the output of controllerR may be avoided, as described.

[0032] If controller R is a digital controller, then the necessaryadaptations may be able to be accomplished quite easily. The content ofintegrator I may be easily read out to perform a calculation inaccordance with the above equation and be utilized, for example, innumerical control NC to calculate correction value Korr. In the samecomputing cycle, in which gain factors KP, KI are reduced, correctionfactor Korr may be loaded into integrator I, so that no abrupt changeoccurs at the output of the speed controller.

What is claimed is:
 1. A method for actuating a holding brake of anelectric drive having a motor and control loop using adaptable controlparameters, comprising: adapting at least one control parameter suchthat, in accordance with an engaged holding brake, oscillations are oneof avoided and greatly reduced; and actuating the holding brake.
 2. Themethod according to claim 1, wherein the control parameters are adaptedin the adapting step such that no change occurs at an output of thecontrol loop.
 3. The method according to claim 1, further comprisingawaiting a compensated state before the control parameters are adaptedin the adapting step.
 4. The method according to claim 2, furthercomprising feeding the control loop a correction value that avoids achange at the output of the control loop caused by the adapting of thecontrol parameters.
 5. The method according to claim 4, furthercomprising changing content of an integrator of the control loop by thecorrection value such that an output of the controller is not changed bythe adapted control parameters.
 6. The method according to claim 1,further comprising: reducing a proportional gain factor and an integralgain factor in the control loop before the holding brake is engaged; andincreasing the proportional gain factor and the integral gain factorafter the holding brake is released.
 7. The method according to claim 6,wherein the proportional gain factor and the integral gain factor arereduced in the reducing step in one of an rpm and a speed controller. 8.A numerical control for a machine tool, comprising: a controller; amotor; a holding brake; and arrangement configured to perform a methodfor actuating the holding brake using adaptable control parameters, themethod including the steps of: adapting at least one control parametersuch that, in accordance with an engaged holding brake, oscillations areone of avoided and greatly reduced; and actuating the holding brake. 9.The numerical control according to claim 8, wherein the controlparameters are adapted in the adapting step such that no change occursat an output of the control loop.
 10. The numerical control according toclaim 8, wherein the method includes awaiting a compensated state beforethe control parameters are adapted in the adapting step.
 11. Thenumerical control according to claim 9, wherein the method includesfeeding the control loop a correction value that avoids a change at theoutput of the control loop caused by the adapting of the controlparameters.
 12. The numerical control according to claim 11, wherein themethod includes changing content of an integrator of the control loop bythe correction value such that an output of the controller is notchanged by the adapted control parameters.
 13. The numerical controlaccording to claim 8, wherein the method includes: reducing aproportional gain factor and an integral gain factor in the control loopbefore the holding brake is engaged; and increasing the proportionalgain factor and the integral gain factor after the holding brake isreleased.
 14. The numerical control according to claim 13, wherein theproportional gain factor and the integral gain factor are reduced in thereducing step in one of an rpm and a speed controller.
 15. A numericalcontrol for a machine tool, comprising: a controller; a motor; a holdingbrake; and means for performing a method for actuating the holding brakeusing adaptable control parameters, the method including the steps of:adapting at least one control parameter such that, in accordance with anengaged holding brake, oscillations are one of avoided and greatlyreduced; and actuating the holding brake.
 16. The numerical controlaccording to claim 15, wherein the control parameters are adapted in theadapting step such that no change occurs at an output of the controlloop.
 17. The numerical control according to claim 15, wherein themethod includes awaiting a compensated state before the controlparameters are adapted in the adapting step.
 18. The numerical controlaccording to claim 16, wherein the method includes feeding the controlloop a correction value that avoids a change at the output of thecontrol loop caused by the adapting of the control parameters.
 19. Thenumerical control according to claim 18, wherein the method includeschanging content of an integrator of the control loop by the correctionvalue such that an output of the controller is not changed by theadapted control parameters.
 20. The numerical control according to claim15, wherein the method includes: reducing a proportional gain factor andan integral gain factor in the control loop before the holding brake isengaged; and increasing the proportional gain factor and the integralgain factor after the holding brake is released.
 21. The numericalcontrol according to claim 20, wherein the proportional gain factor andthe integral gain factor are reduced in the reducing step in one of anrpm and a speed controller.